When an energy-storing phosphor (e.g., stimulable phosphor, which gives off stimulated emission) is exposed to radiation such as X-rays, it absorbs and stores a portion of the radiation energy. The phosphor then emits stimulated emission according to the level of the stored energy when exposed to electromagnetic wave such as visible or infrared light (i.e., stimulating light). A radiation image recording and reproducing method utilizing the energy-storing phosphor has been widely employed in practice. In that method, a radiation image storage panel, which is a sheet comprising the energy-storing phosphor, is used. The method comprises the steps of: exposing the storage panel to radiation having passed through an object or having radiated from an object, so that radiation image information of the object is temporarily recorded in the panel; sequentially scanning the storage panel with a stimulating light such as a laser beam to emit stimulated light; and photoelectrically detecting the emitted light to obtain electric image signals. The storage panel thus treated is subjected to a step for erasing radiation energy remaining therein, and then stored for the use in the next recording and reproducing procedure. Thus, the radiation image storage panel can be repeatedly used.
The radiation image storage panel (often referred to as energy-storing phosphor sheet) has a basic structure comprising a support and a phosphor layer provided thereon.
Various kinds of energy-storing phosphor layers are known. For example, the phosphor layer can comprise a binder and energy-storing phosphor particles dispersed therein, or otherwise can comprise agglomerate of an energy-storing phosphor without binder. The latter layer can be formed by a gas phase-accumulation method or by a firing method.
The radiation image recording and reproducing method (or radiation image forming method) has various advantages as described above. It is still desired that the radiation image storage panel used in the method have as high sensitivity as possible and, at the same time, give a reproduced radiation image of as high quality (in regard to sharpness and graininess) as possible.
For the purpose of improving the sensitivity and the image quality, it has been proposed to form the phosphor layer by a gas phase-accumulation method such as vapor-deposition method or sputtering method. For example, in the vapor-deposition method, the phosphor or constitutional material thereof is heated by a resistance heater or electron beam, vaporized and accumulated on a substrate (e.g., metal plate) to prepare a phosphor layer in which a large number of columns of the phosphor stand parallel to each other. Thus prepared phosphor layer consists of only the phosphor without binder, and there are cracks among the phosphor columns. Accordingly, the stimulating light can be applied efficiently enough, and the emission can be collected also efficiently enough to improve the sensitivity. In addition, since the stimulating light is kept from scattering horizontally, an image of high sharpness can be obtained.
WO 02/20868A1 describes that an alkali halide (e.g., CsBr:Eu) phosphor layer formed by the vapor-deposition method is liable to have an uneven thickness because large phosphor particles are often formed to make uneven spots on the phosphor layer. On the basis of this finding, the WO publication proposes that, after at least 30 wt. % of the phosphor used as the evaporation source is deposited on a substrate, the formed phosphor layer be ground with abrasive to give a phosphor layer having even thickness.
As described above, a phosphor layer formed by a gas phase-accumulation method such as the vapor-deposition method consists of phosphor in the form of columns. According to the WO publication, the phosphor column has a diameter of some micrometers on average at its top surface. The phosphor column, however, does not always uniformly grow. For example, a portion of the phosphor column often grows anomalously, and neighboring phosphor columns may fuse and combine with each other. The present applicants have found that, if the anomalously grown or fused phosphor column (anomalous crystal, often referred to as “hillock”) has a larger diameter at its top surface than a pixel size for reading out a radiation image or than an image size in reproducing the image, it causes a point defect to impair quality of the reproduced image and, as a result, to give unfavorable effects to various diagnoses and examinations.
FIG. 1 is an electron micrograph (×150) partly showing a surface of phosphor layer of a conventional radiation image storage panel, and FIG. 2 is another electron micrograph (×500) partly showing a section of the phosphor layer. As shown in FIGS. 1 and 2, in the phosphor layer formed by the conventional vapor-deposition process, some phosphor columns anomalously grow to become an anomalous crystal having a diameter larger than 200 μm at the top surface thereof.
FIG. 3 is still another electron micrograph (×35) partly showing a radiation image reproduced from the conventional storage panel. FIG. 3 indicates that, if the radiation image information is read out from this storage panel (pixel size: 100 μm, image size: 200 μm), the anomalous crystal in the phosphor layer gives a point defect which is unfavorable for medical diagnoses.
This problem is serious particularly in medical radiography for the chest. In radiographic diagnoses of chest, the pixel size for reading out a radiation image from the storage panel is generally 100 μm pitch while the image size for reproducing the image is 200 μm pitch. Accordingly, in the case where the storage panel is used for medical diagnoses of chest, the phosphor column having a diameter larger than 200 μm at its top surface causes serious troubles.